Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, alcohol and alcohol-containing fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. Various engine systems may be used with alcohol fuels, utilizing various engine technologies and injection technologies. Further, various approaches may be used to control such alcohol-fuelled engines to take advantage of the high octane alcohol fuel, in particular to address engine knocking. For example, engine control methods may include adjustment of boost or spark timing in dependence upon the alcohol fuel, and various other engine operating conditions.
However, the inventors herein have recognized that in addition to an inherent octane effect, an injected fuel may also have a charge cooling effect and an engine dilution effect that depends on the composition of the fuel. For example, in addition to a higher octane number as compared to gasoline, alcohol fuels may also have a higher charge cooling effect that is not represented in the octane number. Thus, fuel injections based only on an inherent octane number of a fuel may lead to overconsumption of the fuel, and necessitate more frequent refueling. In the case of fuel blends, the inherent octane number of a fuel may be based on the volumetric composition (e.g., alcohol concentration) of the fuel. However, the non-linear relationship between the volumetric composition of the fuel and the octane number can make calculation of engine control adjustments complex.
Thus, in one example, the above issue may be at least partly addressed by a method of operating an engine comprising, in response to engine knock, retarding ignition spark timing by a first amount and direct injecting a second amount of a knock control fluid, the first amount of spark retard and the second amount of fluid injection adjusted based on a combination of inherent octane of the fluid, dilution effect of the fluid, and evaporation effect of the fluid.
In one example, a vehicle engine may be configured to use a plurality of fluids, including fuels and fuel blends. The engine may include a direct injector for direct injecting a fluid (such as, water and/or a fuel or fuel blend) into the engine cylinder. The engine may further include a port injector for port injecting a fluid (such as, water and/or a fuel or fuel blend) into the engine cylinder. In response to engine knock, an engine controller may address the knocking with at least some spark retard before adjusting a fluid injection. The amount of spark retard and the amount of fluid injection may be adjusted based on engine operating conditions (such as the knock intensity, the engine speed and load, etc.) and further based on a combination of an inherent octane effect of the fluid, a dilution effect of the fluid, and an evaporation effect of the fluid. A port injection may then be adjusted based on the determined direct injection.
In one example, the inherent octane effect of the fluid may be calculated based on the molar composition of the fluid. For example, the fluid may be a fuel blend including at least two constituent fuels, such as gasoline and ethanol. Herein, the molar composition may be determined based on the molar and/or volumetric fraction, molecular weight, and density of each constituent fuel in the fuel blend. In one example, by using a molar composition of the fuel to determine the inherent octane effect of the injected fluid, a linear correlation between the fuel composition (for example, the alcohol content) and the inherent octane effect (for example, the inherent octane number, or RON) may be achieved, thereby simplifying the calculation of engine adjustments based on fuel type. As such, the evaporation effect of the injected fluid may be based on the charge cooling properties of the fluid while the dilution effect of the injected fluid may be based on the charge dilution effects of the injected fluid.
In one example, spark timing may be retarded up to a predetermined timing based on the molar composition of the fluid. For example, where the injected fluid is a fuel blend including alcohol (e.g., ethanol or methanol), the predetermined timing may be retarded further away from MBT as the molar fraction of alcohol in the blended fuel increases. In alternate examples, the injected fluid may be one or more of ethanol, methanol, other alcohols, gasoline, water, and combinations thereof. In each case, the adjustments may be based on a combination of the inherent octane effect, dilution effect, and evaporation effect of the selected fluid. By addressing knock using at least some spark retard before direct injecting a knock control fluid, knock control may be achieved while improving engine fuel economy.
The dilution effect of the injected fluid may also affect the overall engine dilution. Thus, based on the desired engine dilution (as determined by the engine operating conditions and desired torque), and the dilution effect of the injected fluid, an engine operating parameter may be adjusted. In one example, the engine operating parameter that is adjusted may be an amount of exhaust gas recirculation (EGR). The adjustment may include, when the direct injected fluid has a higher dilution effect, decreasing the amount of EGR provided to the engine as the amount of fluid injected is increased. Herein, the injected fluid may be advantageously used to address knock while also providing some of the desired engine dilution. In comparison, when the injected fluid has a lower dilution effect, the amount of EGR provided to the engine may be maintained while the amount of fluid that is injected is increased. Herein, the injected fluid may be largely used to address knock while EGR is used to largely provide the desired engine dilution. In alternate examples, the adjusted parameter may include an engine operating parameter having an effect on the dilution of the engine, such as a variable cam timing (VCT), an amount of valve lift, an engine boost, etc. Herein, as with EGR, as the dilution effect of the injected fluid increases, the amount of dilution due to VCT retard, valve lift, or boost may be decreased while the amount of fluid that is injected is increased. Still other engine operating parameters may also be adjusted based on the amount and dilution effect of the injected fluid. In this way, engine operating parameters may be adjusted based on the dilution effect of an injected fluid to synergize the addressing of engine knock with the providing of a desired engine dilution.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.